Battery charger and method utilizing alternating dc charging current

ABSTRACT

A battery charger is disclosed for use with various batteries, such as automotive and marine-type batteries. In accordance with an aspect of the invention, the charging current is alternated between non-zero DC charging current levels. By alternating the charging current between non-zero DC charging levels, the battery can be charged to a higher capacity (i.e., ampere hours) faster, thus reducing the charging time and at the same time allow the rating of the battery charger to be increased. In accordance with another important aspect of the invention, the technique for alternating the charging current can be implemented in both linear and switched-mode battery chargers.

CROSS REFERENCE TO RELATED APPLICATIONS

The present application claims priority to and the benefit of USprovisional application No. 60/700,059, filed on Jul. 15, 2005.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a battery charger and more particularlya battery charger for use with various types of batteries includingautomotive and marine-type batteries for both linear and switched-modebattery chargers in which the DC charging current is alternated betweennon-zero average DC charging current levels, which allows the chargingtime to be reduced and also increases the capacity of the batterycharger.

2. Description of the Prior Art

Various types of battery chargers are known in the art. The two mostcommon types of battery chargers are known as linear and switched-modebattery chargers. Linear battery chargers provide an output voltage thatis a linear function of the input voltage. Unused charging power issimply dissipated. Switched-mode battery chargers are more efficient.With such switched-mode battery chargers, only slightly more than theinput power required to generate the charging voltage and current isconsumed. to exist with such switched-mode battery chargers.

The charging characteristics of a battery charger are normallyconfigured to match the battery chemistry of the battery to be charged.For example, lead acid batteries, normally used in automotive and marineapplications, are normally charged with constant power, constant currentor constant voltage or combination thereof. Such batteries are known tobe charged with both linear as well as switched-mode battery chargers.U.S. Patent Application Publication No. US 2005/0088144 A1, assigned tothe same assignee as the assignee of the present invention, discloses anexample of a switched-mode battery charger for automotive and marinebattery applications.

Many different considerations affect the selection of a particularbattery chemistry for a particular application. For example, lead acidbatteries are normally used in automotive and marine batteryapplications because of the ability to deliver relatively large amountsof power. In automotive applications, an initial burst of power isrequired to start the engine. In marine applications, such as CoastGuard applications, the battery capacity is an important considerationfor use in buoys, deployed by the U.S. Coast Guard in the oceanssurrounding the U.S. to transmit weather information to mariners. Suchbuoys are also used for navigation.

Battery capacity is normally measured in terms of ampere hours.Theoretically, the ampere hour capacity is the number of hours that thebattery can deliver a specified level of output current. Due to losseswithin the battery, the ampere-hour output capability of a battery isknown to be slightly less than the ampere-hour input.

During charging, it is necessary to charge the battery to itsfully-charged condition without exceeding the voltage, current, ortemperature, which may damage the battery, as specified by the batterymanufacturer. An exemplary battery charging characteristic curve for anexemplary marine battery is illustrated in FIG. 1. In this example, thecharging current is illustrated by the curve 20. The maximum chargingcurrent is limited by various parameters set forth by the batterymanufacturer, such as temperature cut-off (TCO), the rate of change oftemperature with respect to time (dT/dt), current, and other parameters.The battery charging temperature is identified with the curve 22. Thecurve 24 illustrates the battery voltage, while the curve 26 illustratesthe ambient air temperature. In this particular example, the maximumcharging current is limited to a value slightly greater than 40 amperesduring a constant current mode during a time period t₁. Based upon thecharging characteristics illustrated in FIG. 1, the end of a nominalcharging cycle is shown at the point 27. The ampere hours applied to thebattery by the charger can be obtained by integrating the area under thecurve 20. In this exemplary case, the ampere hours input to the batteryis 40.65 Ah.

As mentioned above, due to internal losses within the battery, theoutput capacity of the battery will be slightly lower than 40.65 amperehours. An exemplary discharge curve is illustrated in terms of FIG. 2.As shown, a fully charged exemplary battery in accordance with FIG. 1 isdischarged at 10 amps while measuring the terminal voltage. For anominal 12-volt automotive battery, the battery is discharged at 10 ampsuntil the terminal voltage reached about 10.5 amps, which was about 235minutes, as indicated by the point 29. The output capacity is thus 10amperes X 235 minutes X 1 hour/60 minutes or about 39.2 ampere hours.

There are several problems with known chargers. First, the chargingtimes are relatively long. Second, the charging characteristics of knownbattery chargers require such battery chargers to be rated at relativelylow values.

SUMMARY OF THE INVENTION

The present invention relates to a battery charger for use with varioustypes of batteries, such as automotive and marine-type batteries. Inaccordance with an aspect of the invention, the charging current isalternated between non-zero charging current levels. By alternating theDC charging current between two non-zero charging levels, the batterycan be charged to a higher capacity (i.e., ampere hours) faster, thusreducing the charging time and at the same time allows the rating ofsuch chargers to be used in relatively higher current applications. Inaccordance with another important aspect of the invention, the techniquefor alternating the average DC charging current can be implemented inboth linear and switched-mode battery chargers.

DESCRIPTION OF THE DRAWINGS

These and other advantages of the present invention will be readilyunderstood with reference to the following specification and attacheddrawings wherein:

FIG. 1 is an exemplary charging curve of an exemplary marine battery bya known battery charger.

FIG. 2 is an exemplary discharge curve for the exemplary marine batteryutilized in connection with FIG. 1.

FIG. 3 is an exemplary charging curve of an exemplary marine battery bya battery charger in accordance with the present invention.

FIG. 4 is an exemplary discharge curve for the battery illustrated inFIG. 3.

FIG. 5 is a diagram of an exemplary charging cycle generated by aswitched-mode battery charger in accordance with the present invention.

FIG. 6 is a diagram illustrating exemplary charging cycles generated byboth switched-mode and a linear battery charger, superimposed on thesame graph, in accordance with the present invention.

FIG. 7 illustrates an exemplary schematic for a control board for anexemplary 100-amp switched-mode battery charger, for use with thepresent invention.

FIG. 8 illustrates an exemplary schematic for a power board for the100-amp switched-mode battery charger illustrated in FIG. 7.

FIG. 9 is an exemplary schematic diagram of a control board for anexemplary 60-amp high-frequency switched-mode-type battery charger.

FIG. 10 is an exemplary schematic diagram for a power board for the60-amp battery charger illustrated in FIG. 9.

FIG. 11 is an exemplary schematic for a control board for a linear-typebattery charger.

FIG. 12 is an exemplary schematic of a power board for the batterycharger illustrated in FIG. 11.

FIG. 13 is an exemplary schematic diagram of a control board for alinear-wheeled linear battery charger.

FIG. 14 is an exemplary schematic for the power board for the batterycharger illustrated in FIG. 13.

FIG. 15 is an exemplary flow diagram for a linear battery chargerillustrating the overall control loop.

FIG. 16 is an exemplary flow diagram for a linear or high frequencybattery charger illustrating the main control loop.

FIG. 17 is an exemplary flow diagram for a linear or high frequencybattery charger illustrating a fast charge method in accordance with thepresent invention.

DETAILED DESCRIPTION

The present invention relates to a battery charger and a method for fastcharging a battery and increasing the discharge capacity of a battery.The battery charger is for use, for example, with various types ofbatteries, such as automotive and marine-type batteries, lead acidbatteries, deep cycle batteries, AGM batteries, and other battery typesand can be implemented in both linear and switched-mode battery chargertopologies. In accordance with an important aspect of the invention, theaverage DC charging current is alternated between non-zero chargingcurrent levels. By alternating the average DC charging current betweennon-zero charging levels, the battery can be charged to higher levels,thus providing increased output capacity and can also be charged muchfaster.

An exemplary charging curve for the battery charger, in accordance withthe present invention is illustrated in FIG. 3, while an exemplarydischarge curve is illustrated in FIG. 4. Referring first to FIG. 3, thecharging current is illustrated by the curve, generally identified bythe reference numeral 28. The charging voltage is illustrated by thecurve 30, while the ambient temperature is illustrated by the curve 32.The battery temperature is indicated by the curve 34. As shown, thecurve 28 illustrates that during a first phase of the battery charger,identified as t₁, the average DC charging current waveform is alternatedbetween between two non-zero values with the maximum value a little over40 amperes and the minimum value around 15 amperes. The minimum andmaximum charging current values have periods in seconds, tens ofseconds, etc. Each period of the minimum and maximum charging currentsrepresents the DC average of a number of DC charging pulses which varybetween zero amperes and some some positive DC value. For example,linear battery chargers are known to generate charging current pulses at120 Hertz Switched mode battery chargers are known to generate chargingcurrent pulses at much higher frequencies. Nonetheless, these chargingcurrent pulses, whether generated by a linear battery charger or aswitched-mode battery charger, have a predetermined duty cycle,frequency and magnitude and may be averaged over a specific time periodto determine their average value, for example, as measured by a DCammeter. The present invention is concerned with varying the averagevalue of the charging current pulses for both linear and switched-modebattery chargers between two non-zero values. For example, asillustrated in FIG. 3, the DC average value is varied between 15 amperesand 40 amperes.

As illustrated in FIG. 3, the fully-charged condition is illustrated bythe point 35 at about 10.3 hours. As mentioned above, the total inputcapacity to the exemplary battery is represented by the area under thecurve 28. In this example, the battery was charged with 126.1 amperehours; over three times the input charge to the same type of batteryillustrated in FIGS. 1 and 2.

As mentioned above, the discharge capacity of a battery is slightly lessthan the input charge due to internal losses within the battery. Asillustrated in FIG. 4, the battery was discharged at 10 amperes untilthe terminal voltage reached 10.5 volts. As indicated by the point 38,the battery was considered to be discharged at the point 38, or about646 minutes. The discharge capacity is thus 10 amperes X 646 minutes Xone hour/60 minutes or about 107.7 ampere hours: a 150 percentimprovement over the battery capacity when charged by the known batterycharger as illustrated in FIGS. 1 and 2.

The alternating DC charging waveforms provide several benefits. First,the charger, in accordance with the present invention, allows moreenergy to be pumped into the battery faster, thereby decreasing thecharging time. Secondly, the charger, in accordance with the presentinvention allows the rating of a battery charger to be increased. Forexample, using the technique, in accordance with the present invention,allows, for example, a 15-amp battery charger to be used in a 40-ampapplication.

In accordance with an important aspect of the invention, the DCalternating charging current technique is applicable to both linear andswitched-mode chargers. FIG. 5 illustrates an exemplary average DCcharging current waveform for a switched-mode charger, while FIG. 6illustrates exemplary waveforms for both switched-mode and linearchargers.

As shown in FIG. 5, an exemplary waveform, identified with the referencenumeral 40, alternates between two non-zero average DC current values,10 amperes and 30 amperes, having a period of 270 seconds (420-150) or4′A minutes. As shown, the exemplary waveform provides an average DCcharging current 30 amperes for 75 seconds (240-165) and an average DCcharging current of 10 amperes for 150 seconds (420-270) for a total of225 seconds.

As shown, the current waveform may not be a perfect square wave becauseof the relatively high frequency of the actual DC charging pulses (asmeasured by an oscilloscope). and, instead, may ramp up and ramp downbetween the two non-zero values. For example, as illustrated in FIG. 5,a rising edge of the waveform 40, identified with the reference numeral42, ramps between a first non-zero DC value of 10 amperes to a secondnon-zero DC current value of 30 amperes in one exemplary incrementaltime period, for example, 15 seconds. The falling edge of the waveform40, identified with the reference numeral 44, drops between a nominal 30amps and 10 amps in two exemplary incremental time periods, for example,30 amps.

FIG. 6 illustrates other exemplary charging waveforms. In particular,FIG. 6 illustrates exemplary DC charging current waveforms whichalternate between 15 amperes DC and 40 amperes DC for both switched-modeand linear battery chargers. In particular, the curve 46 illustrates aswitched-mode charging waveform, similar to FIG. 5. The curve 48illustrates exemplary alternating charging waveforms for a linear-typebattery charger. As shown, the waveforms for the linear-type batterycharger are generally square waves in which the rising 50 and falling 52edges of the waveform are virtually perpendicular to the horizontalaxis.

It is to be understood that the principles of the present invention areapplicable to various waveform configurations having various periods. Itshould also be apparent that the alternating DC current chargingwaveforms in a charging cycle need not be uniform. For example, thewaveforms may vary between different upper and lower DC current valueswithin the same charging cycle or may be relatively constant. Also, theperiods of the waveforms may vary within a particular charging cycle.All such embodiments are contemplated to be within the present scope ofthe invention.

Hardware

As mentioned above, the principles of the present invention areapplicable to both switched-mode and linear mode battery chargers.Various exemplary linear and switched-mode schematics are illustrated inFIGS. 7-12. Exemplary switched-mode charger hardware is generallydescribed in U.S. Patent Application Publication US 2005/0088144 A1,published on Apr. 28, 2005, hereby incorporated by reference. FIGS. 7-10illustrate exemplary schematics for switched-mode battery chargers,while FIGS. 11-14 illustrate exemplary schematics for linear-typebattery chargers. In particular, FIG. 7 illustrates an exemplaryschematic for a control board for a 100-amp switched-mode batterycharger. FIG. 8 illustrates an exemplary schematic for a power board forthe 100-amp switched-mode battery charger illustrated in FIG. 7. FIG. 9is an exemplary schematic diagram of a control board for an exemplary60-amp high-frequency switched-mode-type battery charger. FIG. 10 is anexemplary schematic diagram for a power board for the 60-amp batterycharger illustrated in FIG. 9. FIG. 11 is an exemplary schematic for acontrol board for a linear-type battery charger. FIG. 12 is an exemplaryschematic of a power board for the battery charger illustrated in FIG.11. FIG. 13 is an exemplary schematic diagram of a control board for alinear-wheeled linear battery charger. FIG. 14 is an exemplary schematicfor the power board for the battery charger illustrated in FIG. 13.

The principles of the present invention apply to virtually any linear orswitched-mode battery chargers or charging circuits. In general, suchbattery charging circuits including the various battery chargercircuits, illustrated in FIGS. 7-14, all include a power circuit and acontrol circuit. The power circuits under the control of the controlcircuits are configured to deliver predetermined charging currents. Forexample, the switched-mode battery charger illustrated in FIGS. 7 and 8are configured to deliver 2 ampere, 15 ampere and 40 ampere selectablecharging currents and a engine cranking charging current of 100 amperes.FIGS. 9 and 10 illustrate an exemplary switched-mode battery chargerconfigured to deliver selectable charging currents of 2 amperes and 60amperes. FIGS. 11-13 illustrate exemplary linear battery chargers withselectable slow, medium, fast and starting (or cranking) chargingcurrents.

Referring to FIGS. 7-14, each of the exemplary battery chargers includesa number of selector switches, SW1, SW2, and SW3, which enables thedesired charging current to be selected. As will be discussed in moredetail combinations of these switches may be used for special functions,such as manual mode. The exemplary battery chargers may also include adisplay and/or indicating lights to indicate the selected charging modeand optionally may be used to indicate a proper start-up sequence.

Each battery charger, whether linear or switched-mode, includes amicroprocessor or microcontroller, for example, an ST MicroelectronicsModel No. ST6225C, as generally illustrated in FIGS. 7, 9, 11 and 13.The microcontroller is used for controlling the battery charger andexecutes the software discussed below. As part of that control, themicrocontroller monitors whether the status of the switches SW1, SW2 andSW3 and also drives the indicating lights, as will be discussed in moredetail below.

It should also be understood that the principles of the invention arealso applicable to battery chargers that only charge at a singlecharging level. In other words, the principles of the invention areapplicable to battery chargers in which the charging current is notselectable. All of such configurations are considered to be within thescope of the present invention.

SOFTWARE

The source code for the various battery chargers is provided below. Inparticular, the source code identified in the file 6,000-P8.asm and10,000-PA.asm is for the 60- and 100-amp switched-mode battery chargersillustrated in FIGS. 9, 10 and 7, 8, respectively. The source code filesidentified as 71223D.asm; 71224D.asm; and 30060.asm are useful with thelinear battery charger illustrated in FIGS. 11-14.

Exemplary flow charts for the battery charger in accordance with thepresent invention are also illustrated in FIGS. 15-17. Although FIGS. 15and 17 apply to linear battery chargers, except for the step of turningon SCRs, these FIGS. 15 and 17 apply generally to switched-mode batterychargers as well. FIGS. 16 and 18 apply to both linear and switched-modebattery chargers.

Referring first to FIG. 15, an overall control loop is illustrated. Thecontrol loop runs continuously any time the battery charger is poweredup. In particular, step 100 illustrates a power up or reset condition.After power up or reset, the system checks whether the battery chargerhas been placed in a Test Mode in step 102. The test mode may beinitiated, for example, by a depression of a test mode switch (notshown) or depressing a combination of the switches SW1, SW2, SW3,mentioned above. If the battery charger has been placed in a test mode,the system lights up each of the indicating lights (e.g. LEDs) insequence in step 104 and transfers control to the main control loop instep 106, illustrated in FIG. 16.

In step 108, the system checks whether the battery charger has beenplaced in a Manual Mode. The Manual Mode may be selected by a separateswitch (not shown) or by depressing a combination of the switches SW1,SW2, SW3 or by depressing one or more of the switches SW1, SW2, SW3 fora predetermined time period. In a Manual Mode, the battery charger isturned on for a predetermined time period irrespective of whether abattery is connected to the battery charger. If the battery charger isnot in the Manual Mode, the system checks in step 110 whether a batteryis connected to the battery charger by checking whether the voltage ofthe battery charger output terminals is less than a predetermined value,for example less than 0.1 volts DC. If so, the system assumes no batteryis connected to the battery charger terminals and loops back to steo100. If the voltage at the battery charger terminals is greater than,for example, 0.1 volts DC, the system assumes a battery is connected tothe battery charger terminals and proceeds to steps 112, 114 and 116which illustrate various steps in the Main program loop, illustrated inFIG. 16.

In step 116, the system exits the main loop. In step 118, the systemawaits a timer interrupt or a non-maskable interrupt (NMI) Inparticular, the microcontroller measures the incoming AC power line andgenerates a non-maskable interrupt (NMI) in response to a zero crossing.The NMI is used to turn off the SCRs. The NMI also initiates a softwaretimer. When the tinier times out, the the SCRs are turned on in step121. After the SCRs are turned off, the battery voltage is read andstored in step 122. A running average of the battery voltage may also bemaintained in step 122.

FIG. 16 is a flow diagram of the main control loop. Initially, thesystem is initiated on power up or reset in step 124. Upon power up orreset, the battery charger is initialized in step 126 by initializingthe I/O, RAM, idle timer and the tester mode. Once the battery chargeris initialized, any battery displays are initialized in step 128. Nextin step 130, the status of the switches SW1, SW2, SW3 are checked forspecial startup modes. For example, in a linear battery charger, thesystem checks whether the “slow”, “medium” or “fast” charging modes havebeen selected by way of the switches; SW1, SW2, SW3. In step 132, thesystem checks whether the battery charger is in a Power On Self Test(POST) Mode. If not, the system checks in step 134 whether the batterycharger is in a Manual Mode, as discussed above. If not the systementers the main program loop, shown within the dashed box 136. If thesystem determines that the Manual Mode was selected, software flags areset in step 138 before the system enters the main program loop 136.

Alternatively, if the system determines in step 132 that the batterycharger is in the POST Mode, the various indicating lights or LEDs arelit in sequence in step 140. After the LEDs are lit, the system returnsto step 130 and checks the switches SW1, SW2, SW3 for a selected chargerate. Next, the system again checks whether the battery charger is inthe POST Mode, as discussed above. Assuming that the system is not inthe POST Mode or in the Manual Mode, the battery charger enters the maincontrol loop 136.

The entry into the main control loop begins at step 142, where thesystem checks whether the battery charger is in an abort mode, forexample due to temporary loss of incoming AC power, for example. If so,the system proceeds to the Abort Mode (State 8). Next, the system checkswhether the battery charger is in a Manual Mode in step 146. If so, thebattery charger assumes the Manual Mode (State 15) in step 148.

In step 150, the system checks whether the battery charger is coolingdown after an engine start, i.e whether a predetermined time period haspassed since the battery charger provided 100 amperes of current tostart an automobile engine. If so, the system determines the batterycapacity in step 152, for example, in %. After the battery capacity ischecked, the system checks in step 154 whether a battery is connected tothe battery charger terminals, as discussed above, in step 154 andcharges the battery in accordance with the selected charge rate in step168. If not, the system assumes a default charging state (State 0) instep 156.

If the system determines in step 150 that the battery charger is not inan engine cool down mode, as discussed above, the system checks in step158 whether the switches SW1, SW2, SW3 for specific charging rates, i.efast, medium or slow, have been depressed. If so, the Idle Mode, i.e amode when no charging rate is selected, is terminated. Next, in step160, the battery checks whether an engine start mode has been selected.If so, the system proceeds to step 152 and updates the battery capacity.If not, the system proceeds to step 162 and updates any display of thebattery capacity. Afterwards, the system checks in step 164 whether thebattery charger entered the Idle Mode, i.e charge rate switchesde-selected. If not, the system checks in step 166 if the batterycharger is currently charging a battery. If so, the system checks instep 154 whether a battery is connected to the battery charger. If so,the system jumps to the state corresponding to the selected charge ratein step 168. If the battery charger is not in a charge mode, asdetermined in step 166, the system proceeds to step 168 and proceeds tothe appropriate state.

If the system determines in step 164 that the battery charger is in theidle mode, the system checks in step 170 whether the idle time limit hasbeen exceeded. If so, the system proceeds to step 156 enters a defaultcharge state. If the idle time limit is not exceeded, the system remainsin the idle mode unless the battery charger is charging, as determinedin step 166, and a battery is connected to the battery chargerterminals, as determined in step 154. If the battery charger is notcharging, as determined in step 166, the system proceeds to step 168 andjumps to the appropriate state.

FIG. 17 is a flow diagram which illustrates the control of a batterycharger which utilizes the principles of the present invention andprovides a charging waveform at two alternating non zero average DCcharging currents as discussed above. The flow diagram illustrated inFIG. 17 may be configured as a branch from the main program FIGS. 15 and16, whenever one or more of the charge rates, e.g. slow (2 amperes),medium (15 amperes) or fast (40 amperes) are selected by the selectorswitches SW1, SW2, SW3.

As mentioned above, the battery charger in accordance with the presentinvention provides a charging waveform of the charging current atalternating non-zero average DC current values, as discussed above,defining a fast charge mode. In step 174, lower average DC chargingcurrent value is set. Specifically, the duty cycle is set and a timerfor the lower average DC charging current. For example, with referenceto FIG. 5, the duty cycle and time period that the waveform will besupplying an average DC charging current of 10 amperes is set. Next instep 176, the system checks whether the timer for the lower average DCcharging current has timed out. If not, the system checks in step 183whether the batteryvoltage is less than, for example, 14.2 volts. If thebattery voltage is less than 14.2 volts, the duty cycle is set forconstant current in step 186. If the battery voltage is greater than14.2 volts, the system continues charging in step 188.

If the lower rate timer has timed out, the system proceeds to step 178by way of step 180 and sets duty cycle and a timer for the higheraverage DC value. With reference to FIG. 5, this corresponds to the dutycycle and time period that the battery charger supplies 30 amperes. Instep 180, the system first determines that the battery charger is notalready charging at the higher average DC charging current. If thebattery charger is charging at the higher average DC charging current,as determined in step 182, the system checks in step 184 whether thebattery voltage is less than Vmax+0.2 volts, for example. If so, theduty cycle is adjusted for a constant average DC charging current instep 186. Alternatively, if the battery is being charged at the lowerrate, the system checks if the battery voltage has reached 14.2 volts instep 183. If so, the system proceeds to step 186 and adjusts the dutycycle as discussed above. The system then checks whether the timer forthe higher average DC charging level has timed out in step 176. Once thebattery voltage exceeds Vmax+0.2 volts, as determined in step 184, theduty cycle for the lower average DC charging current is set in step 186and the system continues charging in step 188 at the lower average DCcharging current. While charging at the lower average DC chargingcurrent, after the system has detected a battery voltage greater thanVmax+0.2 volts, the system checks whether the battery voltage hasleveled off in step 190. If the battery voltage has leveled off, theduty cycle is adjusted in step 192 for a constant charge voltage, forexample, a constant 15.5 volts DC. The system checks in step 194 whetherthe duty cycle has leveled off. If not, the system checks in step 196whether the duty cycle was set at a minimum. If not, the system loopsback to step 192. If so, the duty cycle is adjusted for a constant floatvoltage, for example, 13.2 volts in step 198.

If the system determines in step 190 that the battery voltage level hasnot leveled off, the system checks in step 200 whether the batteryvoltage=Vmax. If so, the system returns to step 192 and adjusts the dutycycle for a constant charge voltage. If not the system returns to step188 and continues charging.

Obviously, many modifications and variations of the present inventionare possible in light of the above teachings. Thus, it is to beunderstood that, within the scope of the appended claims, the inventionmay be practiced otherwise than as specifically described above.

What is claimed and desired to be covered by a Letters Patent is asfollows:

1. A battery charger comprising: a battery charging circuit forproviding a battery charging current for charging a battery, saidbattery charging current circuit configured to provide average DCcharging values that alternate between two non-zero average DC valuesdefining afast charging waveform having a low average DC chargingcurrent and a relatively higher average DC charging current.
 2. Thebattery charger as recited in claim 1, wherein said battery chargingcurrent is a linear battery charger.
 3. The battery charger as recitedin claim 1, wherein said battery charging current is a switched-modebattery charger.
 4. The battery charger as recited in claim 1, whereinsaid fast charging waveform is a square wave.
 5. The battery charger asrecited in claim 1, wherein said fast charging waveform ramps up fromsaid lower average DC charging current to said higher average DCcharging current.
 6. The battery charger as recited in claim 1, whereinsaid fast charging waveform ramps down from said higher average DCcharging current to said lower average DC charging current.
 7. Thebattery charger as recited in claim 1, wherein said fast chargingwaveform ramps up from said lower average DC charging current to saidhigher average DC charging current and said fast charging waveform rampsdown from said higher average DC charging current to said lower averageDC charging current.
 8. The battery charger as recited in claim 1,wherein the duty cycle of said fast charging waveform varies as afunction of the battery charge.
 9. The battery charger as recited inclaim 1, wherein said battery charger is configured with a pluralityselectable charge rates.
 10. The battery charger as recited in claim 9,wherein said battery charger is configured so that said fast chargingwaveform is automatically generated when one or more of said selectablecharge rates are selected.
 11. The battery charger as recited in claim1, wherein said higher and lower average DC charging currents areconstant over a charging cycle.
 12. The battery charger as recited inclaim 1, wherein said higher and lower average DC charging currents arenot constant over a charging cycle.
 13. A method for charging a batterycomprising the steps of: (a) charging said battery at a first average DCcharging current; and (b) subsequently charging said battery at a secondaverage DC charging current defining a fast charging waveform.
 14. Themethod as recited in claim 13, further including the step of: (c)charging said battery with said fast charge waveform at one or moreselectable charge rates.
 15. A battery charger comprising: a batterycharging circuit configured with a plurality of selectable charginglevels and further configured so that at one charging level the chargingcurrent is comprised of at least two average DC charging currents duringa charging cycle defining a fast charge waveform that is automaticallygenerated when of said selectable charging levels is selected.
 16. Thebattery charger as recited in claim 15, wherein said battery chargingcircuit is a linear battery charger.
 17. The battery charger as recitedin claim 15, wherein said battery charging circuit is a switched-modebattery charger.